The present invention relates to improving images produced by electrostatographic printers and copiers and more particularly, concerns solving the image problems of edge delineation, and leading edge deletion in an image. Such leading edge deletion and placement problems may manifest as phenomena typically referred to as line shrinkage, halo and white gap artifacts. These artifacts are also sometimes referred to as “slow toner”.
In a typical electrophotographic printing machine, a photoreceptor is charged and exposed to record an electrostatic latent image therein. The photoreceptor is advanced to a development station where toner is deposited on the latent image. It has been found that when a toner powder cloud development system is used, large amounts of toner are deposited where there is an uninterrupted expanse of charged area (such as toward the middle of large image shapes), and toner is starved from locations where there is a strong or sudden change in charge (as found with narrow lines or shapes and on the edges of larger shapes). The result is edge displacement. The effect on large shapes causes them to exhibit a defect called halo, which manifests itself most clearly at the interfaces of solid colors. Halo in color systems appears as white lines at interfaces which should otherwise be a perfect match between two colors.
However, there are other sources of halo defects. These include misregistration and color plane-to-plane interactions, particularly on edges as found in plural image forming station architecture systems. One example of a plural image forming station architecture is an image-on-image (101) system in which the photoreceptive member is recharged, reimaged and developed for each color separation, followed by a single transfer of all colors to paper. Any misregistration of the imaging or development of the photoreceptive member will cause halo. Also, color plane-to-plane problems may occur where a prior developed color toner inhibits the development of a subsequent color toner. These various defects are also observable in single color images as edge distortion or displacement and line shrinkage dependent on the size of the printed object. Line shrinkage of course leads to poor line and text quality due to an erosion or shrinkage of the line edges and corners. Various approaches have been devised to solve these halo and slow toner effects.
The following patents appear to be relevant to edge dilation and the enhancement of the edge of image shapes.
In U.S. Pat. No. 4,791,679, Image Character Enhancement Using A Stroke Strengthening Kernal, to Barski, et al., a character stroke is strengthened by processing video image data with a 16×16 kernal, and moving the kernal one pixel at a time through the image. For each pixel position, sections of the kernal, are selectively filled with black pixels in proportion to the number of black pixels in each section, in accordance with a set of predetermined rules.
U.S. Pat. No. 5,483,351, Dilation Of Images Without Resolution Conversion To Compensate For Printer Characteristics, to Mailloux, et al., discloses an image compensation system which provides dilation or erosion of image features using halfbitting or fullbitting in the rendition of bitmap images, especially on a write-white printer. A region of pixels of an image is isolated which includes two or more correctable pixel locations. A set of state determination rules, based on the formation of pixels in the isolated region, is used to determine a corrected binary pixel state for each of the correctable pixels. Corrections for one correctable pixel may be considered in the state determination rules for adjacent correctable pixels. A single enhanced output pixel is provided for each image input pixel, thereby preserving the original image resolution. Performing enhancements on multiple input pixels simultaneously increases the system throughput.
In U.S. Pat. No. 5,218,350, Image Processing Method For Dodging With Softened Edge Transitions, to Bollman, incorporated by reference herein for its teaching, image appearance characteristics for a display may be defined by two data sets, a first standard data set for the image, and a second set for the area to be dodged, where the dodged area is separated from the remainder of the image by a series of edges. The appearance of any particular pixel in the original is determined by the first bit, or control plane, of the image data, the state of which indicates that pixel appearance is determined by one of the first or second data sets. To soften the transition of the image from one data set to another data set, at the dodged area edges, a filter function is applied to the control plane values, acting on areas adjacent the edge, to create a distribution of control plane values over an area adjacent to the edge. Error diffusion, or dithering of the filtered values, is then applied to quantize intermediate values generated by the filter function to legal values giving the illusion that the edge changes gradually from the first data set to the second data set, thereby creating a softened edge effect, using only a single control bit in the image data.
Therefore, a need exists for techniques which will solve these halo and slow toner effects. Further, there exists a demand for increasing the through-put of printing and digital imaging systems without incurring or exacerbating these problems. Thus, it would be desirable to satisfy such needs or demands by solving the aforesaid and other deficiencies and disadvantages.